With an exponential increase in the number of Internet users and an increase in capacity and diversification of contents, such as high-definition images, an increase in transmission capacity is also desired in radio communication. As a high-capacity radio communication system, the use of a millimeter-wave band with which it is easy to secure a wide frequency band is suitable because of less commercial radio stations. Impulse scheme radio transmission is radio transmission using RF pulses as transmission media and is expected to be used for realizing a wide-band radio communication system.
An impulse scheme radio transmitter obtains a high-frequency pulse signal from a low-frequency pulse signal by multiplication, and therefore, the specific band of a bandpass filter is large as compared with that of a conventional transmitter. Further, a local oscillator or a mixer is not necessary and it is possible to use a low-frequency bandpass filter. Consequently, as compared with a carrier wave scheme narrow-band transmitter, the configuration of the radio unit is simple and inexpensive, and therefore, the impulse scheme radio transmitter is expected to be used for realizing high-capacity radio communication exceeding 10 gigabits per second (10 Gbps).
In the impulse scheme radio communication, ON/OFF modulation to transmit a millimeter-wave pulse in accordance with 1/0 of data and envelope detection are performed. The amount of data that can be transmitted per second (transmission rate) is determined by the pass frequency bandwidth of the bandpass filter. The use of the millimeter-wave band (30 GHz to 300 GHz), to which several frequency bandwidths as wide as several to several tens GHz are assigned for the radio communication use by law, is suitable to realize high-capacity communication at several tens gigabits per second (Gbps) with a simple system.
As the transmission unit of the recent impulse scheme radio transmitter, one that uses a so-called bipolar RZ (Return-to-Zero) type short-pulse generator that generates a pulse having the polarity opposite to that of the immediately preceding “1” in order to eliminate the limitation to the transmission power due to the bright line spectrum has been developed.